The invention relates to magnetic resonance (MR) imaging, and more particularly to synchronization of MR imaging data to motion of a patient.
Synchronizing MR images to the motion of a patient, e.g., to the beating of the heart, respiration of the lungs, or motion of a limb, gives the diagnostician images that have a known correspondence to the motion of interest, e.g., to the phase of the cardiac cycle. Such synchronization can be useful for both cine and still MR images. To achieve synchronization, it is necessary to have a timing signal that is indicative of the phase or position of the body. E.g., in cardiac imaging the timing signal might indicate the start of each cardiac cycle.
One technique for providing a timing signal in cardiac imaging is to connect appropriate electrodes to the patient for monitoring the patient""s ECG while MR imaging data is collected. However, the magnetic fields and pulsed magnetic field gradients used in MR can interfere with the collection of the ECG signal. Special algorithms have been developed in an effort to overcome these difficulties. Chia et al., xe2x80x9cPerformance of QRS Detection for Cardiac Magnetic Resonance imaging with a Novel Vectorcardiographic Triggering Method,xe2x80x9d Journal of Magnetic Resonance Imaging, 12:678-688 (2000). Fischer et al., xe2x80x9cNovel Real-Time R-Wave Detection Algorithm Based on the Vectorcardiogram for Accurate Gated Magnetic Resonance Acquisitions,xe2x80x9d Magnetic Resonance in Medicine, 42:361-370 (1999). Applying the ECG electrodes to the patient, and setting up to acquire the ECG data, is relatively complex and time consuming. And it can become necessary during imaging to relocate the electrodes for a viable ECG signal, and this typically requires that MR image acquisition be stopped and the patient withdrawn from the bore of the MR unit.
Synchronizing MR images is of particular value in segmented or interleaved cine imaging, in which the data for each image is derived from different cycles of the motion of interest. For the images to be meaningful, only data from corresponding phases of different cycles should be combined, hence the need to synchronize the imaging data with the motion. For example, in segmented cardiac cine imaging, the k-space lines for each image may come from 14 or more different cardiac cycles occurring during a single breath hold.
Synchronization of segmented or interleaved cine imaging can be done either prospectively or retrospectively. When done prospectively, imaging data acquisition begins in response to the timing signal derived from ECG electrodes, or alternatively from a finger pulse oximeter or other device designed to monitor a physiological signal that is synchronous with the cardiac cycle. Data acquisition typically continues for a fixed time interval, long enough to cover the systolic phase and the beginning of the diastolic phase. Then, there is typically a quiescent period until the next timing signal. Alternatively, the synchronization can be done retrospectively by continuously acquiring imaging data a synchronously with the ECG-based timing signal, while the time each line of data is acquired relative to the last trigger signal is recorded. After the acquisition, the data are assigned to the appropriate phase of the cardiac cycle based on the recorded timing data.
Attempts have been made in the prior art to derive timing information directly from MR data, in order to eliminate the need for an ECG or other additional timing measurement. But these efforts have relied on collecting additional MR data that is not used in producing the MR images.
For example, Spraggins U.S. Pat. No. 4,961,426 and Spraggins, xe2x80x9cWireless Retrospective Gating: Application to Cine Cardiac Imaging,xe2x80x9d Magnetic Resonance Imaging, 8:675-681 (1990) teach acquiring additional xe2x80x9ctiming slicesxe2x80x9d from which a timing signal can be derived. The timing data, in the form of an echo without phase encoding, is interleaved with imaging data acquisition (every other acquisition is timing data), and can be acquired from a different area of the heart than that being imaged (e.g., an area where motion is more visible). Kim et al., xe2x80x9cExtraction of Cardiac and Respiratory Motion Cycles by Use of Projection Data and Its Application to NMR Imaging,xe2x80x9d Magnetic Resonance in Medicine, 13:25 (1990) uses a similar approach, except that the additional data is transformed into the spatial domain (Spraggins had used the frequency domain data directly) to provide a signal representative of a projection of an image slice onto a line oriented along the direction of time data acquisition.
Another approach is found in Vasanwala et al., xe2x80x9cProspective MR Signal-Based Cardiac Triggering,xe2x80x9d Magnetic Resonance in Medicine, 42:82-86 (1999), wherein a special xe2x80x9ctriggering sequencexe2x80x9d is used to acquire velocity encoded data representative of aortic blood velocity. When a triggering event is found, the system switches from the triggering sequence to an imaging sequence.
In the area of respiratory gating, a technique known as navigator gating or navigator echo derives a timing signal from extra, non-imaging data. Ehman and Felmlee, xe2x80x9cAdaptive technique for high-definition MRI of moving structures.xe2x80x9d Radiology, 173:255-263 (1988). Typically a projection perpendicular to the diaphragm is acquired while an edge detection algorithm is used to determine respiratory cycle position.
In general, in a first aspect, the invention features synchronizing MR imaging data with motion of a patient (e.g., the beating of the heart) by extracting timing information from the MR imaging data, itself, rather than relying solely on additional data acquired solely for timing. By deriving the timing information from the MR imaging data, superior image quality is possible. For example, in cardiac imaging, where images are based on data collected during a single breath hold, more of the available time during the breath hold is available for collecting imaging data.
Superior image quality may also result from direct synchronization of the MR data with the motion affecting it, rather than some indirect measure in the form of external physiological signals, or MR signals not used in the image generation.
Clinical productivity is increased because data collection time is reduced, and because less time is required to prepare a patient for the MR study (e.g., because it is not necessary to attach ECG electrodes).
The invention solves the problem of acquiring an ECG signal in the hostile environment of an MR unit. Inability to acquire a reliable ECG signal is a common cause for failed cardiac MRI exams. Costly and complex equipment (e.g., ECG monitoring equipment) is not needed to produce the timing information.
A wide range of body motions can be synchronized, including voluntary motions like muscular contraction or chewing, as well as involuntary movements like respiration. Respiratory motion is a major cause of artifacts and poor image quality in MR scans of the chest and abdomen. Respiratory motion information can be extracted directly from the MR data and, used to synchronize the image data with the quiescent period of the respiratory cycle, avoiding motion artifacts. This allows patients with difficulty controlling their breath (e.g., elderly patients, infants) to have data collected during free breathing, avoiding the common requirement of breath holding for MRI of the chest or abdomen.
Timing signals can be derived from fetal MR data to avoid the complexity of measuring a fetal ECG, enabling the acquisition of high temporal and spatial resolution MR images of the fetal heart, something which to date has not been possible.
Preferred implementations of the first aspect of the invention may incorporate one or more of the following:
Imaging data may be acquired along radial or spiral k-space trajectories, so that timing information may be extracted from frequently collected k-space points at or near the origin. Depending on the method of extraction, the timing information may be acquired from the raw k-space data or from k-space data transformed into the spatial domain. The timing information may be based on the center point of k-space, known as the echo peak of the raw data. It may also be based on a computed 1-dimensional projection or 2-dimensional image by transforming the raw MR data into the spatial domain. More than one projection may be used to permit computation of the center of mass of the image. The projections may be onto a k-space line with an orientation chosen to enhance sensitivity to the motion of interest. Timing information may be based on correlation of low-resolution images, which may be acquired using interleaved data acquisition (e.g. with each interleaf comprising a group of k-space trajectories covering a dispersed region of k-space). The timing information may be extracted from a selected region of the low-resolution image.
The extracted timing information may be processed to provide temporal correspondence with the motion (e.g., a time value representing the time at which the motion begins or the time of another event during the motion). The processing may comprise extracting a peak, phase, or rate of a time varying signal.
The timing information may be used to retrospectively or prospectively synchronize the MR imaging data with the motion. The motion of the patient may be periodic (e.g., the periodic movement of the heart lungs). The timing information may comprise a time-varying signal that varies in value over the period of the motion. The MR imaging data may be segmented cine imaging data.
The timing information may be extracted solely from the MR imaging data or it may be extracted from a combination of MR imaging data and additional non-imaging data.
The method may be performed using an RF coil localized to the portion of the body that is moving (e.g., the RF coil may be localized over the heart).
In general, in a second aspect, the invention features a method MR imaging, comprising applying a pulse sequence that generates RF signals simultaneously on at least a first and a second RF coil; processing the RF signals acquired on the first coil to extract MR imaging data; processing the RF signals acquired on the second coil to extract data other than imaging data; and using the MR imaging data to produce an MR image.
In preferred implementations of this aspect of the invention, one or more of the following may be incporated: The invention may be applied to synchronizing MR imaging data with motion of a patient, in which case the timing data is acquired from the second RF coil, timing information indicative of the motion is extracted from the timing data, and the timing information is used to synchronize the MR imaging data with the motion. There may be a plurality of first RF coils forming an array o coils, and each is sized and positioned primarily for acquiring MR imaging data. The second RF coil may be localized to the portion of the body that is moving. The MR imaging data may comprise cardiac imaging data and the second RF coil may be localized to an area of the chest in the vicinity of the heart. The timing information may be extracted exclusively from the timing data acquired from the second RF coil. The MR imaging data may be acquired exclusively from RF signals acquired from the first RF coil.
In general, in a third aspect, the invention features synchronizing MR fetal cardiac imaging data with motion of the fetal heart by extracting timing information from MR data rather than from an ECG other non-MR signal.
In preferred implementations of this aspect of the invention, one or more of the following may be incorporated: The timing information may be extracted from MR imaging data (i.e., as with the first aspect of the invention). Alternatively, the timing information may be extracted from MR data not used as MR imaging data.
Other features and advantages of the invention will be apparent from the drawings and detailed description, and from the claims.